In order to meet the increasing demand for wireless data traffic since the commercialization of 4G communication systems, the development focus has been on the 5th Generation (5G) or pre-5G communication system. For this reason, the 5G or pre-5G communication system is called beyond 4G network communication system or post Long Term Evolution (LTE) system.
In order to accomplish high data rates, consideration is being given to implementing the 5G communication system on the millimeter Wave (mm Wave) band (e.g., 60 GHz band). In order to mitigate propagation loss and increase propagation distance, the 5G communication system is likely to accommodate various techniques such as beamforming, massive MIMO, Full Dimensional MIMO (FD-MIMO), array antenna, analog beamforming, and large scale antenna.
Also, for throughput enhancement of the 5G communication system, research is being conducted on various techniques such as small cell, advanced small cell, cloud radio access network (cloud RAN), ultra-dense network, Device to Device Communication (D2D), wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), and interference cancellation.
Furthermore, the ongoing research includes the use of Hybrid FSK and QAM modulation and Sliding Window Superposition Coding (SWSC) as Advanced Coding Modulation (ACM), Filter Bank Multi Carrier (FBMC), Non-Orthogonal Multiple Access (NOMA), and Sparse Code Multiple Access (SCMA).
A relay wireless communication system is characterized in that a source device transmits data to a destination device via a relay device. In the relay wireless communication system, the data transmitted by the source device is relayed by at least one relay device to the destination device. At this time, the relay device processes the data received from the source device and relays the processed data, instead of the received data, to the destination device. The relay device may use one of 3 operation schemes.
The first operation scheme is an Amplify-and-Forward (AF), the second operation scheme is a Decode-and-Forward (DF), and the third operation scheme is a Compressed-and-Forward (CF) scheme.
The AF scheme is characterized in that the data received from a previous node is just amplified and relayed to the next node to deliver the data from the source device to the destination device. This is the most basic method for protecting against decoding failure on the data transmitted from the source device to the destination device.
The DF scheme is characterized in that the relay device decodes and then encodes the data received from a previous node and transmits the newly encoded data. In the case of using the DF scheme, each relay device has to perform decoding and encoding operations every time such data is received. Since the relay device transmits the re-encoded data at a power level equal to or greater than a minimum transmission power, the data can be delivered securely from the source device to the destination device.
The CF scheme is characterized in that the data received from a previous node is compressed for delivery. At this time, the compression is achieved by quantization. Accordingly, the relay device decodes the quantized data and then performs quantization for transmission to the next node. In the CF scheme, each relay device also has to decode the quantized data and quantize the decoded data for transmission such that the data is transmitted at a power level equal to or greater than a minimum transmission power, resulting in secure delivery from the source device to the destination device.
In the above-described schemes, a Hybrid Automatic Repeat Request (HARQ) technique is used to correct errors in the transmitted data. There are two methods for applying HARQ to a DF scheme-based relay protocol as follows.
The first method is to use an end-to-end HARQ, and the second method is to use independent HARQ per node. In the following description, the device which transmits data is referred to as “source”, the device for which the data is destined is referred to as “destination”, and the devices which relay the data from the source to the destination are referred to as “relays”.
In the case of using the end-to-end HARQ, the destination decodes the data received from the source via relays and determines whether any packet error is detected. Depending on whether any packet error is detected, the destination may transmit an ACK/NACK to the source via the relays. The source transmits a new packet upon receipt of the ACK or performs retransmission upon receipt of the NACK.
A description is hereinafter made of the method of using the independent HARQ per node. If a packet is received from the source, the relay decodes the packet to determine whether any error is detected. If the packet is decoded successfully, the relay transmits an ACK to the source and forwards the successfully decoded data to the next node, e.g., another relay or destination. In this case, the source may transmit a new packet. However, if it fails to decode the packet, the relay transmits a NACK to the source to request for retransmission.
In the case of using the above-described CF scheme, it is necessary to apply the end-to-end HARQ technique for the relay network or perform HARQ for correcting errors caused by channel uncertainty of the relay-destination link. In this case, the information signaling and data retransmission should be performed through multi-hop links between the source and destination, resulting in an increase of HARQ processing time delay.